CLI Summary mode

[jayvdb]

I am interested in building a summary output mode, which prints a human readable summary of all files, like lines of 'metric: value' so that it is also valid yaml

This sounds a bit like https://github.com/mozilla/rust-code-analysis/issues/163 , but that seems more about plumbing.

IMO it could even be a separate tool that ingests the output of RCA, so that the user can first run RCA, delete some of its output if the existing filters are insufficient, and then run the reporting tool to get the summary.